The present invention relates to surface acoustic wave devices and has for its object to provide surface acoustic wave devices which can substantially overcome the problems encountered in the prior art surface acoustic wave devices, and have zero temperature coefficient and a high electro-mechanical coupling coefficient.
For most applications of surface acoustic wave devices, the most important factor is the attainment of a sufficiently higher degree of temperature stabilization of delay time. An ST-cut quartz is the most typical surface acoustic wave material with a low temperature coefficient. In addition, the crystals of TeO.sub.2 and AlPO.sub.4 have the zero temperature coefficient. However, these ST-cut quartz, TeO.sub.2 and AlPO.sub.4 have an extremely small electro-mechanical coupling coefficient. Therefore, there has been an ever increasing demand for surface acoustic wave device materials which have not only the zero temperature coefficient but also a large or high electro-mechanical coupling coefficient.
To this end, extensive studies and experiments have been made on the composite surface acoustic wave device materials consisting of two materials. For instance, there have been devised and demonstrated the zero temperature coefficient surface acoustic wave devices comprising in combination a material such as LiNbO.sub.3 or LiTaO.sub.3 which has both a high temperature coefficient and a high coupling coefficient and a material such as SiO.sub.2 which has a negative temperature coefficient. (See "STABILITY OF SAW CONTROLLED OSCILLATORS", T. E. Parker et al., 1975 Ultrasonics Symposium Proceedings IEEE Cat No. 75 CHO 994-4SU and U.S. Pat. No. 3,965,444, granted to C. B. Willingham et al.)
FIG. 1 is a graph of the temperature coefficient versus h.k for a Y-cut lithium tantalate substrate (LiTaO.sub.3) when the surface acoustic waves are propagated in the Z-direction. (FIG. 1 corresponds to FIG. 4 in the accompanying drawings of U.S. Pat. No. 3,965,444.) The product h.k (h is film thickness of the silicon dioxide (SiO.sub.2) layer and k is equal to 2.pi./.lambda. where .lambda. is acoustic wavelength) is plotted along the abscissa while the temperature coefficient of delay time is plotted along the ordinate.
FIG. 2 shows the temperature dependence of an SiO.sub.2 /LiTaO.sub.3 oscillator compared to an ST-quartz oscillator. (FIG. 2 corresponds to FIG. 5 of T. E. Parker et al.) The SiO.sub.2 /LiTaO.sub.3 oscillator's frequency is 399.82 MHz while the ST-quartz oscillator's frequency is 310.7 MHz.
However, the surface acoustic wave device of the type described above; that is, the device consisting of the combination of a Y-cut LiTaO.sub.3 upon which the surface acoustic waves are propagated in the Z-direction and SiO.sub.2 have two major drawbacks or problems to be described below. Firstly, the thickness of a SiO.sub.2 film must be made from 40 to 50% of an acoustic wavelength. As a result, the thermal distortions due to the difference in coefficient of thermal expansion between the SiO.sub.2 film and the LiTaO.sub.3 substrate become very pronounced. (See "PSG/LiTaO.sub.3 (x-112.degree.)", Ebata et al., The Proceedings Of The Institute Of Electronics And Communication Engineers Of Japan US-79-25, P25). Secondly, since the SiO.sub.2 film is thick, unwanted waves are generated. That is, because of the double structure of SiO.sub.2 and LiTaO.sub.3, the waves in some mode which would not be generated on the surface of a LiTaO.sub.3 substrate are generated and interfere with the desired waves.
Temperature coefficient compensated surface acoustic wave devices consisting of a Y-cut, Z-direction propagation LiTaO.sub.3 substrate and a SiO.sub.2 film thereon have a relatively large coupling coefficient, so that their applications are easy. However, in practice, they have not been used because of the drawbacks described above.
Even though ST-cut quartz SAW devices have a coupling coefficient which is lower by one order of magnitude than that of the above-described surface acoustic wave devices, they have found wide applications in which the attainment of temperature stability is a critical factor. In addition to the problem that ST-cut quartz has a low coupling coefficient as described previously, it has a further problem that, as seen clearly from FIG. 2, it has a zero first order temperature coefficient, but its second order temperature coefficient is high. As a consequence, even when it is used in a normal temperature range, the frequency change reaches 50 to 100 ppm, which is higher by one order of magnitude as compared with 5 ppm of AT-cut quartz. There has been a strong demand or need for surface acoustic wave devices which have a temperature coefficient comparable to that of AT-cut quartz, but so far there has not been available.